Ceramic and metal matrix composites are common materials that have the ability to tolerate extreme temperatures and high mechanical loads. However, industry often prefers polymeric resin systems as compared to ceramic or metals because polymers in general perform better under fatigue compared to ceramics and are much lighter than most metals. To this end, so-called high performance polymers are often used. These high performance polymers typically have aromatic and/or heterocyclic repeat units along the backbone that in general make the product more rigid. The low mobility of the aromatic/heterocyclic repeat segments provides a high heat distortion temperature or heat deflection temperature (HDT) and excellent heat stability resulting in excellent property retention at elevated temperatures. In addition, higher aromaticity in a polymer will require higher energy to thermally degrade the backbone resulting in more desired char formation during thermal decomposition. HDT is an essential property that allows a particular thermosetting system to be suitable for a specific application. This property is mainly dictated by the crosslink density and chemical structure of the polymer. HDT may be increased by increasing the number of reactive moieties on the polymer or copolymerizable monomer(s) that may be participating in the resin system, affording a higher crosslink density of the finished product. However, it is important to control the amount of crosslinking since the final properties of the product will depend on the crosslink density. As the crosslink density increases, the finished product becomes more brittle making it unusable for certain applications.
It is important to have the most appropriate balance between crosslink density and chemical structure to afford the desired properties when the resins are used alone or combined with fillers, reinforcements, polymers or other additives. Therefore, in addition to HDT, mechanical properties which may include flexural strength, tensile strength, elongation and compression strength are important to prepared products that may find applications in aerospace, automobile, construction, boat building, infrastructure, electric and electronic.
However such high aromaticity may also be a disadvantage due to such polymers having poor solubility in common solvents, as well as difficult processability due to these polymers having high melting points and viscosities. To overcome these problems, the molecular weight of the polymers have high aromaticity is often decreased; unfortunately, this approach results in a significant loss of their final physical properties. To resolve this disadvantage, polymeric intermediates with low molecular weight and reactive functional groups have been developed. The reactive functional groups have the ability to undergo crosslinking reactions and after they are reacted the resin becomes infusible and insoluble improving the ultimate physical properties.
In the last four decades, several thermosetting resins have been developed to handle moderate to high temperatures and tough conditions, providing and appropriate weight to load ratios and cost savings. A wide range of polymers and oligomeric materials are available which include maleimides, cyanate esters, phenolics, aromatic polyethers, polysulfones and polyketones. Some of these materials are able to incorporate HDT, impact strength, toughness, low flammability, hydrolytic stability, chemical and solvent resistance and dielectric properties. The properties accomplished from these materials can provide composite systems that can be used in various applications which can include molding, lamination, infusion, pultrusion, encapsulation, coatings, adhesives, electrical and electronic components.
Much attention has recently been given to thermosetting systems that can be cured thermally or via free radical polymerization to achieve high thermal stability and excellent properties. Among these types of resin systems include polysulfones such as those described in U.S. Pat. Nos. 4,701,514 and 4,806,601. The resins described therein have terminal and/or pendant vinyl containing groups that can be polymerized either thermally or via free radical polymerization. The polymers are aromatic and it is necessary to use polar aprotic solvents that require multiple dissolving and precipitation cycles in order to remove them from the polymerization mixture. The polymers have glass transition temperatures (Tg) from about 170° C. for a molecular weight of about 1000 to a glass transition above 200° C. for molecular weights higher than 5,000.
U.S. Pat. No. 4,665,137 describes the preparation of difunctional polyphenylene oxide. The polymers are prepared with various molecular weights in the range from about 1,000 to about 5,000 containing styrene end groups. The glass transition temperature of these polymeric systems is higher than 200° C. after curing. Multiple steps are necessary for the preparation of these thermosetting polymers including several washing steps to remove the byproducts and unreacted materials.
U.S. Pat. Nos. 6,835,786, 7,388,057, and 7,781,537 describe the preparation of polyphenylene oxide containing terminal and/or pendant methacrylate groups. Various vinyl type monomers are also proposed and provide systems that are cured with peroxide affording crosslinked systems with glass transition temperatures ranging from about 140° C. to over 200° C. Various steps are described in the preparation of the crosslinkable polymers which include solvent processing and removal.
U.S. Publication No. 2011/0172359 and WO2008/119973 describe the preparation and crosslinking of polyfunctional urethanes containing at least three to six ethylenically unsaturated groups in combination with a crosslinkable monomer having at least two ethylenically unsaturated groups. The preparation of these thermosetting systems requires the use of toxic isocyanates that can easily react with moisture and alter their reactivity.
U.S. Publication No. 2011/0207950 describes ethylenically unsaturated crosslinkable systems that are substantially free of any vinyl unsaturated monomer. The reactive components show good properties after crosslinking, however, HDT values are below 120° C.
Thus there remains the need to prepare thermosetting resin systems that may include excellent processability, high HDT, good mechanical properties, curable by a simple process that may include thermal, free radical room temperature polymerization or at moderate temperatures, UV, electron beam or radiation. In addition, it would be advantageous to have a simple and affordable process that would yield products free of potentially toxic or harmful components in the resulting thermosetting materials.